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Premium vs Regular for 2005 Prius

I have read before in several places that there is no difference in gas mileage when using premium or regular gasoline for the Prius. UNfortunately, I have to say that those people are correct.

I ran a test with three tankfuls of premium gasoline. I compared the mpg for the three tanks of premium gasoline with the previous 3 tanks of regular and the following 3 tanks of regular. My car just turned 15,000 mi, so this testing was done previous to this mileage.

Much to my dismay, there was no significant difference. The gas mileage for all nine tankfuls was in the 51-54 mpg range. My driving over the test period was back and forth to work, and some errands in town now and again. I have a 30 mile, mostly highway, trip to work, one way, where I usually set the cruise control between 65 and 72 mph, depending on traffic. The weather was very similar throughout the test period in terms of temperature, sunny/cloudy days, etc. The only thing I'm not sure of is wind direction and magnitude, but that probably averaged out over the time period.

I did not provide the detailed data because there was no difference. If anyone is interested, I can post the numbers later.

One thing I did notice was that the mpg calculated by the car was roughly 1.5 mpg higher than that calculated by using the gallons shown on the gasoline pump. I would expect this to be the case since I would expect the gasoline stations to err on the side of slightly overestimating sales volume.